In recent years optical communication is being widely used for the high-speed transmission of large amount of information. Optical waveguides used in optical communications can be broadly divided into optical fiber and planar type optical waveguides. The prior art includes planar optical waveguides comprising a silicon substrate, lower cladding provided on the silicon substrate, a core provided on the lower cladding, and upper cladding layer provided on the lower cladding, over the core (see Japanese Patent Application Laid Open No. 7-318739).
In an optical waveguide having this composition the refractive index of the silicon substrate is considerably higher than the refractive index of the core, so care must be taken to prevent light leaking from the core to the silicon substrate. For this reason, the lower cladding must be sufficiently thick, 20 μm for example. It results in a long fabrication time.
In contrast, there is another type of planar optical waveguide that comprises a silica substrate, a core provided on the silica substrate and upper cladding layer provided on the lower cladding, covering the core (see Japanese Patent Application Laid Open No. 9-297237). In the case of an optical waveguide having this composition, the silica substrate has a lower refractive index than the core, so the substrate itself can function as the lower cladding. This has the merit of eliminating the need to separately form lower cladding, therefore enabling the waveguide to be fabricated using fewer fabrication steps.
However, a number of problems have been reported when a core is formed directly on the silica substrate, such as that birefringence is produced, increasing the polarization dependency. Various methods have been proposed for improving the polarization dependency and other characteristics even when using a silica substrate, such as by interposing a buffer layer between the silica substrate and the core (see Japanese Patent Application Laid Open No. 10-142436 and Japanese Patent Application Laid Open No. 2002-189139).
Japanese Patent Application Laid Open No. 2003-14959 discloses a method of decreasing the polarization dependency in an optical waveguide that uses a silicon substrate, by prescribing the range of the relationship between the thermal expansion coefficient of the upper cladding layer and the thermal expansion coefficient of the silicon substrate, and the relationship between the thermal expansion coefficient of the upper cladding layer and the thermal expansion coefficient of the lower cladding.
However, even when a buffer layer is interposed between the silica substrate and the core, birefringence has arisen, caused by differences between the thermal expansion coefficients of the core and the buffer layer, or between the thermal expansion coefficients of the core and the upper cladding layer. Such problems have been particularly pronounced when the upper cladding layer is subjected to fluidization annealing to eliminate voids between adjacent cores.